Pregel Programming API

When implementing the init and compute methods in UserPregelAlgorithm, users mainly rely on the PregelContext object, which provides the following APIs:

**Table 1** PregelContext API
Method and Attribute	Description	Remarks
ext_id(nid)->int	Obtains the user-defined external ID of the current vertex. Only IDs that can be converted to the int type are supported.	Basic operations on graph data (values and topologies)
value(nid)->int/float/bool	Obtains the value of the current vertex.
set_value(nid, new_value)->None	Sets the value of the current vertex.
out_edges(nid) -> List[int]	Obtains the list of outgoing edges of the current vertex.
edge_dst(eid)->int	Obtains the target vertex of the current edge.
num_nodes	Obtains the number of vertices in a full graph.
num_edges	Obtains the total number of edges in a full image.
send(dst_nid, msg)->None	Sends messages to the target vertex.	Data exchange operation between vertices
halt(nid) ->None	Sets the current vertex to the halt state.	When all vertices in the graph are in the halt state and there are no messages, or when supersteps reach the maximum number of iterations, the algorithm terminates.
superstep -> int	Obtain the current number of supersteps.